Process for producing a carbon-particle structure

ABSTRACT

A process for producing a carbon-particle structure, including the step of irradiating opaque carbon dioxide at and/or near its critical point, as light is scattered, with a UV-wavelength laser beam to produce a carbon-particle structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.11/597,231 filed Nov. 21, 2006, now issued as U.S. Pat. No. 7,807,025which was the National Stage of International Application No.PCT/JP2005/010001 filed May 25, 2005, the entireties of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a novel method of decomposing carbon dioxide,which is useful for solving carbon dioxide environmental problems, and anovel method of forming a carbon particle structure by thedecomposition.

BACKGROUND OF THE INVENTION

Carbon dioxide reduction is a worldwide important task to beaccomplished from the viewpoint of global environmental problems.However, effective methods have not been developed for the decompositiontreatment of carbon dioxide discharged, for example, from livingenvironment and industrial environment.

Carbon dioxide: CO₂ is a compound of carbon and oxygen. Accordingly, ifCO₂ can be decomposed and utilized as a novel carbon resource, then agreat contribution to the society could be realized.

The current situation, however, is that studies on carbon dioxidedecomposition and effective utilization of carbon dioxide as a carbonresource have not hitherto been made very eagerly.

SUMMARY OF THE INVENTION

In view of the above, an object of the present invention is to provide anovel technical method that can realize carbon dioxide reduction, whichis a task to be urgently accomplished from the viewpoint of globalenvironmental problems, that can realize carbon dioxide decompositionfor the carbon dioxide reduction, and that can realize the utilizationof carbon resources obtained from the carbon dioxide with a higher addedvalue.

The object of the present invention can be attained by a method fordecomposing carbon dioxide, comprising the step of irradiating carbondioxide in a supercritical or subcritical state (i.e., at and/or nearits critical point) with a UV-wavelength laser beam to decompose carbondioxide.

Secondly, the object of the present invention can be attained by aprocess for producing a carbon-particle structure, comprising the stepof irradiating carbon dioxide in a supercritical or subcritical statewith a UV-wavelength laser beam to produce a carbon-particle structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the construction of an apparatusused in the working example.

FIG. 2 is a diagram showing an image that exemplifies an SEM image of acarbon particle structure produced in Example 1.

FIG. 3 is a diagram showing an image that exemplifies another SEM imageof the carbon particle structure produced in Example 1.

FIG. 4 is a diagram showing an image that exemplifies still another SEMimage of the carbon particle structure produced in Example 1.

FIG. 5 is a diagram showing an image that exemplifies a further SEMimage of the carbon particle structure produced in Example 1.

FIG. 6 is an image that exemplifies the results of an EDS analysis ofthe carbon-particle structure produced in Example 1.

FIG. 7 is a diagram showing an image that exemplifies an SEM image of acarbon particle structure produced in Example 2.

FIG. 8 is a diagram showing an EDS image of the carbon particlestructure shown in FIG. 7.

FIG. 9 is a diagram showing a TEM image of the carbon particle structureshown in FIG. 7.

FIG. 10 is a diagram showing an electron beam diffraction image of thecarbon particle structure shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The best mode for carrying out the invention having the above featureswill be described.

In the decomposition of carbon dioxide and the production of a carbonparticle structure according to the present invention, carbon dioxidethat is in a supercritical or subcritical state in relation to itscritical point (critical density 466 kg/m³, critical pressure 7.38 MPa,critical temperature 304.2 K) is irradiated with a UV-wavelength laserbeam. Various laser beam sources and optical system can be properlyadopted for this irradiation. Typical examples of UV(ultraviolet)-wavelength laser beams include YAG-THG (tertiary highfrequency): wavelength 355 nm, YAG-FHG (quaternary high frequency):wavelength 266 nm, and KrF excimer: wavelength: 248 nm.

In the UV-wavelength laser beam irradiation, the laser beam may not befocused, or alternatively may if necessary be focused.

Further, any proper apparatus may be used for bringing the carbondioxide to a supercritical or subcritical state.

In the production of the carbon-particle structure, a substrate may beused, and examples of substrates usable herein include substrates formedof various metals such as aluminum, nickel, tungsten, molybdenum,magnesium, silver, gold, tin, titanium, tantalum, and silicon, variousmetal alloys such as stainless steel, nickel-base alloys, and magnesiumalloys, or inorganic materials such as alumina, graphite, BN and SiC.Alternatively, these substrates may not be used.

As exemplified also in the working example which will be describedlater, the carbon particle structure is produced as a single particulatebody or is produced in such a state that a plurality of particulatebodies have been aggregated, fused or bonded together. The size of thesecarbon particle structures is very small and is usually not more thanseveral tens of micrometers (μm), for example, in the range of severaltens of nanometers (nm) to several tens of micrometers (μm). This sizecan be regulated by irradiation energy, for example, the wavelength ofthe UV-wavelength laser or irradiation time.

Further, a carbon particle structure having a concave on its surface, ahollow carbon particle structure, or a semispherical or partiallyspherical carbon particle structure can also be produced.

At the present stage, the method and process according to the presentinvention are considered based on the following mechanism.

Near the critical point of carbon dioxide (critical density 466 kg/m³,critical pressure 7.38 MPa, critical temperature 304.2 K), largemolecule clusters are produced and the structure becomes opaque(critical opaline luster) as light is scattered. Due to this nature,various unique behaviors regarding physical properties are exhibited.For example, the specific heat or compressibility diverges asapproaching the critical point.

Upon exposure of carbon dioxide near the critical point to UV laser, itis considered that, when photons collide against C—O bond, the C—O bondis cleaved. That is, a carbon dioxide decomposition process (CO₂→C+O₂)is considered to take place stochastically. In particular, as describedabove, since large molecule clusters are formed near the critical point,it is considered that the probability of collision of photons againstCO₂ increases and, consequently, the decomposition process (CO₂→C+O₂) ispromoted. Carbon atoms are bonded to each other to form a nanostructureor microstructure. It is considered that, as the temperature differencefrom the critical point increases, the probability of collision ofphotons against CO₂ molecules is reduced, resulting in a reduced CO₂decomposition rate.

It is needless to say that the above consideration will be furtherdeepened and developed.

The following Examples further illustrate the present invention.

However, it should be noted that the present invention is not limited tothe following Examples.

Example 1

FIG. 1 is a schematic diagram showing an apparatus used in this Example.The apparatus is constructed so that a substrate is disposed inside ofthe apparatus and a laser beam is applied through a glass window. Carbondioxide was introduced into this apparatus, and the carbon dioxide wasirradiated near the critical point (atmosphere temperature 31.4° C.)with a 266 nm-wavelength UV laser beam. As a result, the decompositionof carbon dioxide and the production of a carbon-particle structure wereobserved.

FIGS. 2, 3, 4 and 5 exemplify SEM images of the produced carbon particlestructure. FIG. 6 exemplifies the results of an EDS analysis of thecarbon-particle structure produced on an aluminum substrate, indicatingthat the resultant structure is a carbon-particle structure.

It was found that the production of the carbon-particle structure isobserved in the case where an aluminum, graphite or other substrate isused, as well as in the case where no substrate is used.

For comparison, carbon dioxide with one atom of pressure was irradiatedwith the above laser beam. As a result, the production of acarbon-particle structure could not be observed at all. In this case,when an aluminum substrate was used, nano-size aluminum spheres wereproduced.

Example 2

In the same apparatus as in Example 1, a silicon substrate is disposedwithin the apparatus, carbon dioxide near the critical point wasirradiated with a 266 nm-wavelength UV laser beam without focusing. As aresult, it was confirmed that a carbon particle nanostructure wasproduced on the silicon substrate.

FIG. 7 exemplifies an SEM image of the resultant carbon particlestructure deposited on a horizontal surface and a vertical surface ofthe silicon substrate. FIG. 8 is an EDS image corresponding to FIG. 7 inwhich the green part shows carbon atoms.

FIG. 9 shows a TEM image of a carbon particle structure, and FIG. 10shows an electron beam diffraction image of the carbon particlestructure, indicating that the carbon particle structure is an amorphousstructure.

Example 3

The procedure of Example 2 was repeated, except that each of nickel,magnesium, and carbon substrates were used instead of the siliconsubstrate. In this case, the UV wavelength laser beam was used forirradiation as in Example 2. As a result, it was confirmed that, in allof these substrates, a carbon particle structure was produced.

Example 4

The procedure of Example 2 was repeated, except that a 248 nm-wavelengthUV laser beam was used for irradiation instead of the 266 nm-wavelengthUV laser beam. Also in this case, it was confirmed that a carbonparticle structure was produced.

INDUSTRIAL APPLICABILITY

The first embodiment of the present invention can realize thedecomposition of carbon dioxide at or around room temperature. UVwavelength laser irradiation does not cause a temperature rise upondecomposition. According to the second embodiment of the presentinvention, upon the decomposition of carbon dioxide as a carbonresource, a carbon particle structure useful as a functional material invarious fields such as electric, electronic, medical, catalyst,lubricant, plastic, and mechanically molded product fields can beproduced. The production of the carbon particle structure is alsopossible without the use of any substrate.

1. A process for producing a carbon-particle structure, comprising thestep of irradiating opaque carbon dioxide at and/or near its criticalpoint, as light is scattered, with a UV-wavelength laser beam to producea carbon-particle structure.
 2. The process for producing acarbon-particle structure according to claim 1, wherein thecarbon-particle structure is produced on a substrate.
 3. The process forproducing a carbon-particle structure according to claim 1, wherein thecarbon-particle structure has an amorphous structure.
 4. A process forproducing a carbon-particle structure, comprising the steps of:irradiating opaque carbon dioxide at and/or near the critical point, aslight is scattered, with photons; and colliding the photons with thecarbon dioxide to produce the carbon-particle structure.
 5. The processfor producing a carbon-particle structure according to claim 2, whereinthe carbon-particle structure has an amorphous structure.
 6. A processfor producing a carbon-particle structure, comprising the steps of:irradiating large molecule clusters of opaque carbon dioxide produced atand/or near the critical point, as light is scattered, with photons; andcolliding the photons with the opaque carbon dioxide having largemolecule clusters to produce the carbon-particle structure.